#include<iostream>
using namespace std;
int main()
{
	int n;
	cin >> n;
	int a[21] = {};
	a[1] = 1;
	a[2] = 2;
	for (int i = 3; i <= n; i++)
	{
		a[i] = a[i - 1] + a[i - 2];
	}
	cout << a[n];

	   
}